1. Field of the Invention
The present invention relates to a graphite material coated with a silicon carbide film, and, more particularly, to a graphite material coated with silicon carbide which can be suitably used for various parts in an apparatus for drawing-up single crystals of silicon by the CZ method, for example.
2. Description of Background Art
Single crystals of silicon used for the manufacture of ICs and LSIs have conventionally been produced by the CZ method. As shown in FIG. 1, in an apparatus for drawing-up single crystals of silicon by the CZ method, silicon polycrystals are placed in a high purity quartz crucible 4 and melted by heating using a graphite heater 8. A seed crystal 1 (a single crystal) attached to the top of a drawing axis is dipped into a molten silicon polycrystal fluid 3 and slowly drawn up while being rotated around a rotation axis 7, whereby the molten silicon polycrystal fluid 3 is coagulated and a single silicon crystal 2 is grown at the bottom edge of the seed crystal 1.
Because the quartz crucible 4 has insufficient strength and becomes soft at a high temperature, the quartz crucible 4 is reinforced by being engaged in a graphite crucible 5. In addition, a graphite material which is stable under high temperature conditions is used for a gas rectification pipe 9, a heat shielding member 10, a thermal insulation pipe 11, and the like. In this manner, many graphite members are used in the apparatus for drawing-up single silicon crystals.
In the operation for drawing up a single silicon crystal, molten silicon and quartz react in the apparatus according to the following reaction formula (1) to produce SiO gas.
Si+SiO2xe2x86x922SiO (g)xe2x80x83xe2x80x83(1)
The SiO gas produced reacts with the graphite material in the apparatus and is converted into SiC gas, while generating CO gas.
SiO (g)+2Cxe2x86x92SiC+CO (g)xe2x80x83xe2x80x83(2)
In this manner, the graphite material in the apparatus is gradually eroded by SiO gas. The CO gas produced is dissolved in molten silicon and increases the content of carbon in the drawn-up single silicon crystal. This impairs the characteristics of the single silicon crystal as a semiconductor. In addition, SiC produced on the surface of the graphite material causes fine cracks, resulting in a short life for the material.
As a method for overcoming these problems, a technology of suppressing the reaction between the graphite material and SiO gas by covering the graphite material with silicon carbide has been known. For example, Japanese Patent Application Laid-open No. 58-172292 discloses a heat generating body for the apparatus for drawing-up single silicon crystals which comprises a silicon carbide layer formed over the surface of the graphite material. Japanese Patent Application Laid-open No. 58-172295 discloses an apparatus for drawing-up single silicon crystals which has a silicon carbide layer provided between a quartz crucible and a graphite crucible.
Japanese Patent Application Laid-open No. the following rewritten paragraph: 62-138386 discloses an apparatus for drawing-up single silicon crystals by the CZ method, which is provided with a radiation screen with a shape similar to a reverse cone surrounding the single crystal rod, with the bottom opening proximate to the molten liquid surface, and the upper end being in the shape of an outwardly expanding triangle or a circular rim configuration curved like an arrow which covers the top of the crucible. The inner and outer surfaces of the radiation screen is made from silicon carbide, silicon nitrite, or boron nitride, and the intermediate layer has a thermal insulating complex layer structure made from a carbon felt.
Japanese Patent Application Laid-open No. 5-221758 discloses a graphite material coated with silicon carbide by chemical vapor deposition, wherein the silicon carbide film thickness of the region where cracks are easily produced is 60-90 xcexcm, which is 50-75% of the average film thickness of the entire film.
Silicon carbide (SiC) coating the graphite material reacts with SiO gas according to the following reaction formula (3). However, because the reaction is initiated at about 1820xc2x0 C., which is higher than the melting point of silicon (1420xc2x0 C.), the reaction of the formula (3) does not take place when single silicon crystals are drawn up, thereby preventing the surface of the graphite material from deteriorating.
SiC+SiO (g)xe2x86x922Si+CO (g)xe2x80x83xe2x80x83(3)
Therefore, coating a graphite material member with silicon carbide prevents the reaction of graphite and SiO gas, and ensures stable operation of the apparatus. However, this involves an increase in the manufacturing cost. Specifically, although the thicker the SiC film, the more stable the operation of the apparatus in the SiO gas, the manufacturing cost increases almost in proportion to the SiC film thickness.
A graphite material coated with silicon carbide usually comes to the end of its useful life by attachment of splashed molten silicon while the drawing-up operation of single silicon crystals is repeated. Apart from this, the apparatus may become unusable due to damage such as cracks and peeling in SiC films which are caused by physical impact from handling errors or the like during assembling operations or maintenance of the parts.
SiC films generally having a high resistance against physical impact increase in strength as the film thickness increases. Conventionally, SiC films with a thickness of about 60-150 xcexcm are used. However, a thin film is preferred as a matter of course from the viewpoint of decreasing the manufacturing cost.
Accordingly, an object of the present invention is to provide a graphite material coated with silicon carbide which can overcome the above-mentioned contradictory problems. Another object of the present invention is to provide a graphite material coated with silicon carbide which can be manufactured at a low cost and exhibits a high physical impact resistance.
To achieve the above objective, the graphite material coated with silicon carbide of the present invention comprises a graphite substrate coated with a silicon carbide film with a thickness of 30-50 xcexcm, which is characterized in that there is a mixed layer of graphite and silicon carbide with a thickness of 10-500 xcexcm from the interface of the graphite substrate and the silicon carbide film through the inside of the graphite substrate and further that the silicon carbide film has a peel strength of 5 Mpa or more. The silicon carbide film in the present invention is attached by the chemical vapor deposition (CVD) method and is suitably used as a material for an apparatus for drawing-up single silicon crystals.